The cloud system center is defined as the focal point of all the curved
lines or bands of the cloud system. It can also be thought of as thepoint
toward which the curved lines merge or spiral.

Procedure:

(1) The CSC is located at the center of the eye or at the center of curvature
of a partial eye wall when one of these features is observed.

(2) When the CSC is not obvious, locate the model expected CSC.

Draw a line along the "curved band axix" through the most dense
(coldest) por-tion of the bandm The
axis should roughly parallel the concave (inner) over-castboundary
of the band. Locate the model expected center location inrelation
the the curved band. (See plus symbols in diagram in Step 2A.) Thecenter
is located near the inner (concave) edge of the band on the coun-terclockwiseend (comma head) portion of the band.
Locate tightly curvedlines,
merging lines, or CDO near the point where the center is expected tofall.
The CSC is located at the center of curvature, near the point ofmergence
or at the center of the CDO (for CDO of _< 1 1/2° latitude in size).For
large CDO’s, the center is sometimes defined by an arc of overshootingcloud
tops or in an isolated cluster of convective tops. When not visible,use
(3) below.

(3) When features are not visible at the expected CSC, or when the curved
band is not apparent, use the circle method. The method consists offirst
drawing lines following the cloud line curvature or curved boundariesthat
fall within the curve of the curved band axis, and then fitting circlesto
the lines with tightest curvature.The CSC is located at the center of thearea common to the circles. For relatively
circular embedded center patternsof
>T3.5 intensity, fit a log 10° spiral overlay to the curved band axis tolocate center.

(4) When a cloud minimum wedge is visible on the concave side of the band
near its middle, the CSC is located at the midpoint of a line drawn bet-weenthe deepest cloud minimum incursion of the
wedge and the counterclockwiseextremity
of the curved band axis. This method is frequently used with EIRpictures.
In EIR pictures, the center is often located in the tight gradientnear
the coldest part of the pattern.

(5) When the location of the CSC is unclear, or could be placed at different
locations, use all the methods above along with an extrapolation fromthe
past track positions in making the final decision.

(6) When more than one well-defined CSC is apparent, use the one defined
by the strongest appearing, lowest level cloud lines that best fitsthe
past track of the storm. When strong vertical shear is apparent, rememberthat the upper level (dense) clouds will
not be centered directly over the low-levelcenter,
but will be displaced with the CSC on the tight temperaturegradient
(sharp boundary) side of the dense cloud pattern.

Step 1A. Initial Development

The earliest signs of tropical cyclone development are observed about 1
1/2 days before a disturbance reaches
tropical storm strength. At this time,the
disturbance is classified a T1. A T1 is first used when a cluster ofdeep
layer convective clouds showing line or band curvature has the followingthree
properties.

(1) It has persisted for 12 hours or more.

(2) It has a cloud system center defined within an area having a diameter
of 2 1/2° latitude or less which has persisted for 6 hours.

(3) It has an area of dense, cold (DG or colder) overcast* of >1 1/2°
in extent that appears less than 2° from the center. The overcast mayalso
appear in cumulonimbus lines the curve around the center.The
cloud system center will be defined in one of the followingways:

(1) Curved band, a dense (DG or colder) overcast band that shows some
curvature around a relatively warm (cloud minimum) area. It should curveat
least one-fifth the distance around a 10° log spiral. Cirrus, when visible,will indicate anticyclonic shear across
the expected CSC. (See diagrams, Step6,
PT 1.5 pattern types.)

(3) Curved low cloud lines showing a center of curvature within 2° of
a cold (DG or colder) cloud mass. (See diagrams, Step 2B, DT 1.5 pattern.)In many cloud clusters that eventually
develop, the northern boundariesshow
a straightening about 1 1/2 days prior to the T1 classifications. Duringthe
organizing stage of the T1 pattern, there may be extreme variability in thecloud pattern. In most developments at the
T1 stage, strong upper-level hori-zontalanticyclonic
shear will be indicated across the disturbance center whencurved
cirrus lines are present to reveal the shear. These upper level clouds.may
indicate patterns far more advanced than T1 at the time of the initialclassification.
These patterns do not involve deep tropospheric circulationsat
this time and will be short lived. This means that the Day-2 data T-numbermay at times be less the Day-1’s, but
still development is indicated as longas
theDT is 2 or more. There may also be times during the first two days ofdevelopment
when cirrus or convective clouds are almost absent, showing littlepattern
during the nighttime hours. This usually does not mean the storm isweakening.
The rule is to never lower the T-number at night during the first24
hours of development. A flat boundary rotating clockwise across the northside of the pattern throughout the period
is a good sign of development. Notethat
a classification of T1 forecasts tropical storm

*The amount of cold overcast may decrease during the subsequent nighttime
hours making it crucial that the
analyst watch for the required amount of overcastwhen
it occurs.intensity (T2.5) 36
hours after the T1 observation only when the environmentis
expected to remain favorable. A minus symbol is used after the T1 to indi-catea T1 pattern that is not expected to
develop. (See step 11. )

STEP 2. DETERMINE THE PATTERN TYPE THAT BEST DESCRIBES YOUR
DISTURBANCE AND MEASURE CLOUD FEATURES AS INDICATED

The manner in which the cloud system center is defined determines the pattern
to be analyzed. The pattern types listed below are described onthe
following pages. When the cloud pattern being analyzed does not resembleone
of the patterns, proceed to Step 3.

Step 2A. "Curved Band" Pattern

Step 2B. "Shear" Pattern

Step 2C. "Eye" Pattern

Step 2D. Central Dense Overcast (CDO) Pattern

Step 2E. Embedded Center Pattern

General Analysis Rules:

1. When short-interval pictures are available, use the average measure-ment
of all of the pictures with well-defined
features taken within the 3 hourperiod
ending at analysis time.

2. When two or more T-number estimates are made from the same picture,
use the estimate closest to the MET.

3. When in doubt concerning ambiguous features, bias the analysis toward
the MET.

Step 2A. Curved Band Pattern

The intensity estimate determined from this pattern type is derived by
measuring the arc length of the curved
band fitted to a 10° logarithm spiraloverlay.
(A circle will give the same answer most of the time.) The inten-sityvalues
that relate to the curved band length are given in the analysisdiagrams,
Figure 1,3. Curved band measurements may be used with both VIS andEIR
pictures until an intensity of DT 4.5 is reached. For EIR patternsgreater
than DT3.5 use measurements from VIS diagram.

The spiral overlay is fitted to the curvature of the dense (cold) band by
first drawing a line along the
"curved band axis" and then fitting the spiralcurve
to the line drawn. The curved band axis is defined as the axis of thecoldest
overcast gray shade (most dense clouds) within the cloud band. Theline
should roughly parallel the overcast edge on the concave side of theband.
When the band indicates two possible axes, use the one with tightestcurvature.
Cellular cold globs that do not fall in line with the curve of thecomma
band are ignored when drawing the line. Fit the spiral to the linedrawn
on the picture and measure the spiral arc length of the dense (cold)band
that follows the spiral curve.

In EIR patterns (like those in Figure 2, Step 6, Row b), the cold comma
band will often show warm breaks through
its middle. These breaks will appearto
be almost clear in the VIS picture, When this occurs, draw the comma axisas though it were continuous through the
breaks paralleling the edge of thecloud
minimum incursion into the concave side of the band. As the curved bandpattern
evolves it will usually be defined by the dark gray shade of the BDcurve,
but may at times appear defined in warmer or colder shades of gray. Attimes
the boundaries of the band must be interpreted from its form in previouspictures.

During the first 2 days of development (T1 to T2), the amount of overall
band curvature may change excessively, very little, or even decreasesomewhat
for short periods even though typical development is occurring. Forthis
reason, the tendency should be to raise the T-number by one during thefirst
24 hours of development as long as the band remains curved enough for T2and
clear signs of weakening or rapid development are not apparent. It is alsoimportant to allow at least 24 hours to
pass between a T2 and a T4 classifica-tion.Even
though the coiling process has been observed to be faster than thisat
times, the surface pressure does not fall accordingly.

During the T2.5 or T3 stage, a tightly curved band < 1 1/4° diameter
of curvature observed within the curve of the broad curved band canalso
be, used as an indicator of tropical storm intensity. This is evidence thatthe wall cloud is forming. This tight
curvature at weak tropical storm inten-sityis
often ragged in appearance but will have deep-layer convective cloudi-nesson nearly opposite sides of a system
center.

Step 2B. Shear Patterns

Shear patterns appear in pre-hurricane stages of development when vertical
shear prevents the cold clouds from bending around the cloud systemcenter
as they do in the curved band patterns. The pattern may also appearafter
the hurricane stage has weakened to a pre-hurricane pattern because ofincreasing
vertical shear.

The intensity estimate determined from this pattern type is derived by (1)
the way in which the cloud system center
is defined and (2) the distancebetween
the low cloud center and the dense, cold overcast. For shear patternsassociated
with tropical storm intensity (T2.5 to T3.5), the center will bedefined
by parallel, circularly curved low cloud lines with a diameter of about1.5°
latitude or less. They indicate a center either near the edge or underthe
edge of a dense, cold (DG or colder) overcast cloud mass (see patterns inStep 2B, Figures 1,3. During the weaker
stages of development (T1.5 + .5),the
low cloud center will either be poorly defined in spiral lines within 1.25°of the cold overcast, circularly defined
but some distance (>1.25° latitude)from
the cold overcast clouds, or circularly defined near a small amount (<11/2° diameter) of dense overcast.